No Trick of the Eyes

While the green flash of sunset it hard to spot, it is real

By J. Alex Knoll

We usually focus on the darkened sky in this space, but these late summer sunsets provide a chance to glimpse a strange solar phenomenon. Simply called green flashes, these are bursts of light as the sun crosses the horizon line. Those who’ve seen it describe a green-colored, flame-like burst as the sun winks from sight. Perhaps you’ve already witnessed it and chalked it up to your eyes playing tricks.
This is no trick of the eye, however, but rather sunlight passing through heavy, humid air, which bends and refracts the rays of light. As the sun sets lower, the angle of refraction increases, shifting through the spectrum of colors like light through a prism, with green being the last visible color to flash through and above the horizon line.
To see the green flash, you need a clear view of the western horizon, and better yet, a view over water or sprawling flatlands. Timing, too, is crucial. You must look just before the sun disappears — too late and you miss the flash; too early and your eyes will be blinded, perhaps irreparably so.
After sunset, Mercury appears above the west-northwest horizon. So close to the sun, Mercury seldom strays far from its blinding glare. But at magnitude –1, it is far brighter than you might expect from such a small object. The moon reaches first-quarter the 7th, when it forms a loose triangle with Saturn and Spica. The next night, the moon is just a few degrees below Spica.
Monday, the waxing gibbous moon is just above red Antares, the heart of Scorpius, while Wednesday it is among the stars of teacup-shaped Sagitarius.
Jupiter rises around 2am and is high in the southeast with the approach of dawn. Mars rises around 4am, followed by Venus, still lighting the way for the coming sun.